In many aircraft, the main propulsion engines not only provide propulsion for the aircraft, but may also be used to drive various other rotating components such as, for example, generators, compressors, and pumps, to thereby supply electrical and/or pneumatic power. However, when an aircraft is on the ground, its main engines may not be operating. Moreover, in some instances the main propulsion engines may not be capable of supplying the power needed for propulsion as well as the power to drive these other rotating components. Thus, many aircraft include one or more auxiliary power units (APUs) to supplement the main propulsion engines in providing electrical and/or pneumatic power. An APU may also be used to start the propulsion engines.
An APU is, in most instances, a gas turbine engine that includes a combustion system, a power turbine, and a compressor. During operation of the APU, the compressor draws in ambient air, compresses it, and supplies compressed air to the combustion system. The combustion system receives fuel from a fuel source and the compressed air from the compressor, and supplies high-energy combusted air to the power turbine, causing it to rotate. The power turbine includes a shaft that may be used to drive a generator for supplying electrical power, and to drive its own compressor and/or an external load compressor.
The combustion system in an APU may include a combustor, a plurality of fuel injectors, one or more fuel manifolds, and a high-pressure pump. These combustion system components can be relatively expensive to manufacture and install. Moreover, the fuel injectors may foul due to coking of the small fuel passages that extend through the injectors. This fouling can necessitate injector cleaning, which can be costly and time consuming. Fuel injector fouling can also cause hot streaks in both the combustor and downstream hot section, which can reduce the overall life of the combustor and the downstream hot section, and an uneven temperature profile in the APU, which can cause hot corrosion of, and/or thermal fatigue to, the turbine. These latter effects can also increase system operational and ownership costs.
Hence, there is a need for an APU that is both durable and reliable, and that can be fabricated and operated at reduced costs relative to current APUs, by eliminating most, if not all, of the above-noted drawbacks associated with present APU combustion systems. The present invention addresses one or more of these needs.